It is known to carry out this operation by employing a tool connected in a drill-string and positioned substantially in the vicinity of the top of the liner. The tools known to the art provide a means for circulating fluid through the length of a drill string to the lower end of the liner and which can also redirect the cleaning fluid at higher flow rates out of the drill string into the casing above the liner.
Such tools have been made of at least two generic types. The more common of these utilises the well known practice of dropping spherical balls or darts down the drill string to open or close valves, thereby alternating the circulation path of the fluid. There are a number of disadvantages associated with this type of circulation tool, including the length of time taken for the balls to fall from the surface to the tool which renders it difficult to co-ordinate the arrival of the ball in its required position with the arrival of the cleaning fluid. The alternate flow rates of the fluid relative to the casing and liner clean out must also be so co-ordinated.
Yet further, it is usually necessary to repeat the cleaning out of the liner and casing a number of times with different cleaning fluids. In order to do this there must be a sufficient number of spherical balls and associated valves and ports to redirect the clean-out liquid flow on each occasion either from the lower end of the liner to the upper end or from the upper end to the lower end.
Yet further, it is possible for the balls or darts not to engage at their targeted location, causing malfunction.
The second type of circulation tool is a weight set tool, an example of which is described in our earlier British Patent Number 2 272 923. This type of circulation tool dispenses with the need for drop balls or darts and allows for an unlimited number of recirculation stages. The tool permits circulation fluids to separate regions in a borehole by increasing or decreasing the load exerted on at least part of the tool in the borehole. This also enables redirection of the circulation flow without any significant time delay.
With this second type of circulation tool, a tubular assembly is connected to the drill string and is provided with a generally axial fluid outlet and a generally transverse fluid outlet, and an obturating member which is moveable between a first position closing the transverse fluid outlet and a second position at which the transverse fluid outlet is open. The obturating member is moved relative to the tubular assembly by extending or collapsing the tool, the latter movement occurring by causing a shoulder coupled to the obturating member to engage with a formation in the borehole.
Typically, when the tool is in the extended position the obturating member or an extension thereof is keyed or otherwise engaged with the tubular assembly so that torque applied through the tubular assembly is transmitted to the obturating member. That is to say there is negligible relative rotational movement between the tubular assembly and the obturating member when the tool is in the extended position, thereby allowing the tool to function in a manner such that apparatus below the tool may be rotated.
On the other hand, when the tool is in the collapsed position, typically with the generally transverse outlet open, the obturating member is disengaged from the tubular assembly allowing for relative rotation between the two components. While the obturating member is so disengaged, torque can not be transmitted down the drill string beyond the circulation tool.
Thus, in the past it has been necessary with such weight set circulation tools to mutually engage a mating portion such as a key or spline of the obturating member or an extension thereof with a corresponding mating portion of the tubular assembly in order to close the generally transverse outlet.
A disadvantage of this design in that on some occasions the respective mating portions may fail to properly engage, preventing the full extension of the tool and leaving the transverse outlets from undesirably. Where this occurs the drilling apparatus is disenabled and it is necessary to remove the entire drill string from the borehole, lending to substantial downtime and associated cost.